Microphone apparatus of the type including vacuum tubes used for converting a sound signal to an electric signal have been produced and used in the past. In most of these microphones the vacuum tubes have now been replaced with semiconductors, however, such vacuum-tube microphones are still used to provide a delicate tone quality different than the tone quality provided by semiconductor microphones.
One problem that occurs with the vacuum-tube type microphone apparatus is that the vacuum tube heats the interior of the microphone housing, so that the temperatures of the electric parts contained in the microphone housing are increased. Because these electric parts all have different specific heats, the electric parts exhibit different temperatures as they are being heated to a steady-state temperature. For this reason, the temperature dependant characteristics of the electric parts cannot be stabilized until the temperatures of all of the electric parts are stabilized.
Another problem is that during vacuum tube operation, thermions are emitted from the cathode with increasing plate temperature because of plate losses and heat radiation from the heater. The potential between the plate and the cathode accelerates the thermions toward the plate, and the accelerated thermions collide with the plate to emit secondary electrons so that the space between the grid and the plate becomes filled with stray electrons. Such stray electrons impede the electron flow from the cathode toward the plate and result in noise in the plate current. The stray electrons flow toward the wall surface of the glass envelope of the vacuum tube and thereby charge the glass envelope. When the electrons are further attracted, gases are emitted to reduce the extent of vacuum within the glass envelope of the vacuum tube, so as to impede the electron flow from the cathode toward the plate. As a result, noise is introduced onto the plate current.
A third problem is that the vacuum tube is subject to thermal failure and/or mechanical breakage, because the glass envelope is heated to such a high temperature.